1. Field of the Invention
This invention relates to an image heating device using electromagnetic induction heating and an image forming device using the same. More specifically, the present invention relates to an image heating device used in image forming devices, such as electrophotographical devices or electrostatic recording devices, that is suitable as a fixing device for thermally fixing unfixed toner, and to an image forming device using the same.
2. Description of the Prior Art
As image heating devices, for which thermofixing devices are a typical example, contact-heating devices such as heat-roller devices and film-heating devices are used conventionally.
In recent years, due to the demand for shorter warming-up periods and reduced energy consumption, there have been attempts to use electromagnetic induction heating, which generates heat with high efficiency and allows concentrated heating, for the heat source of these contact-heating image heating devices.
FIG. 10 shows an image heating device of the film-heating type, which is a typical example of a device using electromagnetic induction heating for the heat source (see Publication of Unexamined Japanese Patent Application No. Hei 7-114276). As is shown in FIG. 10, a magnetization coil 203 is wound around a core material 202 on the inner side of a rotating endless film 201. Using this coil, an alternating magnetic field can be caused to penetrate the film 201. Then, this alternating magnetic field induces an induction current in the film 201, which serves as heat-generating material and as heating material, and due to the heat generated by the induction current in the film 201, a toner image 206 is fixed on a recording material 205, which passes between the film 201 and a pressure roller 204. Numeral 207 in FIG. 10 denotes a thermistor for detecting the surface temperature of the pressure roller 204. Depending on the temperature detected by this thermistor 207, the current applied to the magnetization coil 203 is regulated. In this example, a special layering structure is devised for the film 201, so that the heat generated by the film 201 does not transmit as easily towards the side of the magnetization coil 203.
Including this conventional example, image heating devices using magnetic induction heating generally can heat necessary parts intensively and with high efficiency, so that they are useful as one means for reducing warming-up periods and saving energy.
However, in order to effectively reduce warming-up periods and save energy, it is necessary to reduce the thermal capacity of the heat-generating member or the heating member in addition to making the heating means more effective, which brings about new problems.
When the thermal capacity of the heat-generating member or the heating member is reduced, the temperature of the heat-generating member or the heating member reacts with sensitivity to changes in the generated heat or the escaping heat, which promotes temperature changes. Moreover, it is useful to reduce their thicknesses in order to reduce the thermal capacity, but then also their internal thermal conductivity worsens, so that partial temperature differences arise easily, and it becomes difficult to regulate the temperature of the entire heat-generating member or heating member to a uniform and stable temperature. The above-noted conventional image heating device using film-heating is an example where this problem is particularly apparent.
Moreover, in the regular film-heating method, the thermal capacity of the film is set as small as possible to reduce the warming-up period, but this gives rise to the problem that the film temperature partially becomes too high. When the film temperature becomes too high, the heat generation becomes unstable, and hot offset may occur, which in turn causes the destruction of the film and the components around it. Taking the conventional image heating device in FIG. 10 as an example, this problem is aggravated when a recording material 205 whose width is smaller than the width of the image heating device in the depth direction of the drawing is continuously being transported. This means, heat is dissipated into the recording material 205 at the portion where the recording material 205 is transported, so that the heating has to be performed correspondingly, but if portions where no recording material 205 is transported are heated simultaneously, the temperature in these portions will rise, because the thermal capacity of the film is small and the thermal conductivity in the width direction is poor. Then, when the temperature of the film partially becomes excessively high and a recording material 205 with broad width is transported, hot offset occurs, or the overall amount of heat generated becomes unstable, which in turn may result in damage of the magnetization coil 203, which provides heat generation. It is not possible to regulate such a partial temperature rise by detecting the temperature only in the film serving as the heat-generating member and heating member or other members in the above-described conventional example.
On the other hand, when the entire amount of heat generated is limited to prevent temperature rises, the temperature at the portions with high temperature absorption will drop, which may bring about insufficient fixing at these portions.
Not only in the film-heating method, but also when reducing the thermal capacity in the heat-roller method using a halogen lamp or magnetic induction by reducing the thickness of the roller in order to reduce the warming-up time, the same problems arise because of the instability of the generated heat and because of partial overheating and underheating. On the other hand, in the above-noted publication, an attempt was made to achieve temperature self-regulation using a film whose Curie temperature has been adjusted, but according to our experiments, it is difficult to achieve suitable temperature self-regulation using a heat-generating member (film) with that structure. In other words, in this example, the electrically conductive film is formed considerably thinner than the skin depth, and the cross-sectional area of the path where the induction current flows is the same above and below the Curie temperature, so that the amount of heat generated above and below the Curie temperature is almost the same. Consequently, with this conventional configuration, it is impossible to perform a suitable temperature regulation for the image heating device, so that it cannot solve the problem of partial temperature rises and drops.
One of the results of the research which lead to the present invention was that to achieve effective temperature self-regulation applicable for an image heating device, it is necessary that (i) during start-up, a large amount of heat is generated by letting almost the entire induction current flow through a highly resistive portion, (ii) once the Curie temperature is exceeded, the amount of heat generated is decreased by letting more induction current flow through a portion with low resistivity, and (iii) certain conditions should be satisfied so that the difference between these amounts of heat generated exceeds a certain value. Furthermore, to achieve optimum fixing, there is a certain range within which the temperature to be regulated has to be.